Pursuant to a congressional request, GAO reviewed the National Aeronautics and Space Administration's (NASA) estimate for the cost to operate the International Space Station after assembly is completed, focusing on: (1) whether any space station-related costs are not included in NASA's estimate; (2) the level of uncertainty in the cost estimate for operations, especially with regard to the potential impact of changes in Russian participation; and (3) how NASA funding requirements will be reduced by sharing costs with international partners or through commercial use and operations.

GAO noted that: (1) NASA's $1.3 billion estimate does not include all funding requirements related to space station operations; (2) additional items that will have to be funded in the future within the space station budget include costs for upgrading obsolete systems and operating an alternative propulsion module; (3) NASA has not developed detailed estimates for potential upgrades to combat component obsolescence and improve performance, but space station officials believe that a robust enhancement program could cost $100 million or more per year; (4) NASA has not estimated the cost of operating an alternative propulsion module being procured to provide reboost if Russia is unable to provide that function; (5) items that GAO determined to be space station-related that are funded in other NASA budget lines include space shuttle flights, civil service personnel, principal investigators, and space communications; these are estimated to cost a total of $2.5 billion in fiscal year 2004; (6) there is a high degree of uncertainty in NASA's estimate for the cost to operate the space station from 2005 to 2014; (7) NASA's original estimate of $13 billion for operating the space station was developed to aid in evaluating life-cycle costs of redesign options rather than to accurately forecast budget needs; (8) this estimate did not consider end-of-mission costs for either extending the life of the space station beyond 10 years or decommissioning it; (9) adding to the uncertainty of future costs, the full impact on operations if Russia is unable to fulfill its obligations is not known at this time; (10) NASA would incur costs to operate an alternative propulsion module, but does not yet know whether there will be a shortfall in Russian logistics flights or how such a shortfall would be spread among the shuttle and international partner resupply vehicles; (11) there is insufficient information at this time to determine the amount that NASA funding requirements could be reduced by international partners' contributions toward common operating costs; (12) in sharing operating responsibilities for the space station, NASA and Russia have agreed to exchange services rather than funds; (13) however, NASA and Russia may not be able to achieve a balance in the services provided to each other if Russia cannot fulfill its obligations; (14) NASA's share of common operating costs has increased and could change again if international partners revise their participation in the space station program; and (15) allowing partners to pay common costs with services may not reduce NASA funding requirements.